Cancer is the second leading cause of death in the United States. Chemotherapy remains a critical cancer treatment modality, but often produces only marginal benefits due to the development of resistance. Our long- term goal is to develop an efficacious approach to improve cancer chemotherapy and to overcome cancer chemoresistance. Our overall objective in the present application is to develop a novel strategy to enhance the efficacy of gemcitabine against pancreatic tumors and to overcome pancreatic tumor resistance to gemcitabine by combining the gemcitabine-nanoparticles previously developed in our lab with a novel method to breach the tumor desmoplastic stroma. Gemcitabine as a single agent is the first line treatment of advanced pancreatic cancers. However, tumors acquire resistance over time, which becomes a major issue for most gemcitabine- related therapies. Pancreatic tumor resistance to gemcitabine arises from both biological and physiological barriers. We have shown that our gemcitabine-nanoparticles can overcome the biological barrier. However, data from a number of recent studies demonstrate the essential role of tumor desmoplastic stroma as a physiological barrier, preventing gemcitabine or other drugs from reaching tumor cells. Our central hypothesis is that combining the delivery of gemcitabine using our gemcitabine-nanoparticles with the breaching of tumor stroma by depleting tumor-associated macrophages will effectively overcome pancreatic tumor resistance to gemcitabine. We plan to test the hypothesis by pursuing the following specific aims: (1) to elucidate the mechanisms underlying the gemcitabine-nanoparticle's ability to overcome gemcitabine resistance; (2) to evaluate the antitumor activity of the gemcitabine-nanoparticles in animal models that closely resemble human pancreatic cancers; (3) to identify the extent to which modulating pancreatic tumor desmoplastic stroma will affect the antitumor activity of the gemcitabine-nanoparticles and their ability to overcome gemcitabine resistance. The current proposal combines the expertise of a pharmaceutical scientist experienced in drug delivery (Cui) and two cancer biologists with expertise in pancreatic cancer models (Hursting) and tumor histology (Kiguchi). The primary innovation of this work is the targeted depletion of tumor-associated macrophages to breach the tumor desmoplastic stromal barrier, allowing our gemcitabine-nanoparticles to then efficiently reach and kill tumor cells, even cells that are resistant to gemcitabine. This highly significant work addresses the urgent need to overcome pancreatic tumor resistance to chemotherapy, particularly gemcitabine. Study findings will provide the proof of principle that when combined with a novel and clinically feasible method to breach tumor desmoplastic stroma, our gemcitabine-nanoparticles can improve the clinical outcomes of pancreatic cancer therapy. Consequently, these studies will provide the initial underpinnings for future clinical trials testing this strategy and will also encourage the testing f similar strategies with other chemotherapeutic agents, as well as the broader application of similar strategies in other tumors.